mips: clean-up in hardware detection (Cavium/DSP ASEs)

This change is a clean up in MIPS hardware detection code.
New flag for Cavium Company ID is added, as well as the codes for 34K and
74K processors (MIPS Company ID). The later two represent platforms with DSP
ASEs implemented (Rev 1 and Rev 2 respectively). Macros to detect these two
platforms have been added as well.

Additional macros to extract Company ID out of hwcaps added as well, and
used where possible.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2764 8f6e269a-dfd6-0310-a8e1-e2731360e62c